题目
Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.
Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().
解题思路
代码
class Solution:
def calculate(self, s: str) -> int:
stack = []
sign = 1
res = cur = 0
for c in s:
if c.isdigit():
cur = cur * 10 + int(c)
else:
res += cur * sign
cur = 0
if c == '+':
sign = 1
elif c == '-':
sign = -1
elif c == '(':
stack.append(res)
stack.append(sign)
res = 0
sign = 1
elif c == ')':
res += sign * cur
res *= stack.pop()
res += stack.pop()
cur = 0
return res + cur * sign
